A Normal type Pokemon that is always seen in a Pokemon Center helping a Nurse Joy.
Happiny, Chansey's Pre-Evolved Form
Chansey is a helping and healing type Pokemon that is commonly seen with Nurse Joy throughout the Pokemon series. However, in later generations other Pokemon are used to replace Chansey, so it is commonly seen more in the earlier adaptions. Some of those Pokemon include but not limited to Happiny and Blissey. It is a Normal-type that can use a variety of attacks from double-slap to egg bomb. It is believed to be used in medicine in the Pokemon world due to its compassion and caring nature, a rare trait among most Pokemon.
Nurse Joy, Chansey's Boss
The newest of those replacements is Audino, which is used in the Unova region. While Chansey's are used in Pokemon Centers across the following regions: Hoenn, Kanto, Johto, and Sinnoh. It is commonly accepted that the change occurred due to the sudden movement and reboots of the entire Pokemon world.